In general, to hold given image quality of an X-ray image output from a sensor, an X-ray fluoroscopic system monitors the dose of X-rays which have reached the sensor, and controls the incident dose to make the value of the X-ray dose constant. If the dose which has reached the sensor is low, an instruction to increase the incident dose is issued, and if the dose which has reached the sensor is high, an instruction to decrease the incident dose is issued, thereby controlling the irradiation amount of X-rays. In this specification, control of the dose will be referred to as automatic dose control (ADC) hereinafter.
If fluoroscopic images of the same object are continuously obtained by successively capturing X-ray images, the fluoroscopic images of the object are continuously obtained with a constant dose after optimizing the irradiation amount of X-rays by ADC. This state will be referred to as a state in which “ADC is stable” hereinafter. In the state in which ADC is stable, the tone of an X-ray image observed by the user remains unchanged, and an image free from any luminance fluctuation is displayed.
Since, however, an X-ray generation apparatus cannot output a completely constant dose, an output which fluctuates to some extent is generally obtained. As a result, even though fluoroscopic images of the same object are continuously obtained, the luminance of an X-ray image observed by the user may fluctuate. To cope with this, the X-ray image is optimized by further performing image processing for the image obtained by optimization by ADC, thereby suppressing luminance fluctuations. For example, an input image is analyzed to create a tone conversion curve so that a region of interest always has a constant luminance value, and a result of processing the image using the created tone conversion curve is output as a final image.
To generate an optimum tone conversion curve for suppressing fluctuations of the X-ray generation apparatus, it is necessary to perform image analysis to determine the state of the X-ray image. In such image analysis, an object region is extracted by extracting a collimation region and direct irradiated region (region where X-rays directly enter) from the X-ray image, and an analysis value for setting an optimum tone of the extracted object region is calculated. By improving the analysis accuracy, it is possible to output a more optimum final image.
However, as the analysis accuracy improves, processing becomes more difficult, resulting in an increase in probability that image analysis fails. If image analysis fails, it is impossible to obtain an optimum tone conversion curve, so an image having an inappropriate luminance is output. If fluoroscopic images of the same object are continuously obtained, a frame for which image analysis has failed has an inappropriate luminance, and the user feels that flicker has occurred.
As a method of correcting the influence of flicker, there is provided a method of reducing the influence of a failed analysis value by smoothing, in a time-axis direction, analysis values calculated by image analysis (PTL 1). As another method, there is provided a method of analyzing an image to calculate an analysis value, and adding, to the analysis value, a difference value between the analysis value and an analysis value obtained by smoothing in a time-axis direction (PTL 2). A value for stabilization is calculated by smoothing in the time direction, and a value is made to match the calculated value, thereby eliminating flicker.